Germanium-based quantum well devices

ABSTRACT

A quantum well transistor has a germanium quantum well channel region. A silicon-containing etch stop layer provides easy placement of a gate dielectric close to the channel. A group III-V barrier layer adds strain to the channel. Graded silicon germanium layers above and below the channel region improve performance. Multiple gate dielectric materials allow use of a high-k value gate dielectric.

RELATED APPLICATION

The present application is a Divisional of U.S. patent application Ser.No. 12/655,468, filed on Dec. 30, 2009, entitled “GERMANIUM-BASEDQUANTUM WELL DEVICES”.

BACKGROUND Background of the Invention

Most integrated circuits today are based on silicon, a Group IV elementof the Periodic Table. Quantum well transistors based on non-siliconmaterials may provide superior device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device.

FIG. 2 is a cross sectional side view that illustrates a buffer regionthat is formed on the substrate.

FIG. 3 is a cross sectional side view that illustrates anotherembodiment of a germanium quantum well channel transistor device.

FIG. 4 is a cross sectional side view that illustrates anotherembodiment of a germanium quantum well channel transistor device.

FIG. 5 is a cross sectional side view that shows a material stack thatmay be used to form another embodiment of a germanium quantum wellchannel transistor device.

FIG. 6 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device.

FIG. 7 is a cross sectional side view that illustrates the etch stopregion present in some embodiments.

FIG. 8 is a cross sectional side view that illustrates what portion ofthe etch stop region may be altered in an embodiment.

FIG. 9 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device, according to another embodimentof the present invention.

FIG. 10 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device, according to another embodimentof the present invention.

FIG. 11 is a cross sectional side view that illustrates multiplesubregions of a buffer region that comprises a group III-V material.

FIG. 12 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device that has one or morenon-homogenous barrier and/or spacer regions.

FIGS. 13 through 20 are graphs that illustrate bottom barrier and spacerregion material compositions according to various embodiments of thepresent invention.

FIG. 21 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device that has multiple gate dielectricregions.

FIG. 22 is a cross sectional side view that illustrates a device with agermanium quantum well channel region transistor and a group III-Vmaterial quantum well channel region transistor on the same substrate.

FIG. 23 is a cross sectional side view that illustrates a device with agermanium quantum well channel region transistor and a non-quantum wellchannel transistor on the same substrate.

DETAILED DESCRIPTION

In various embodiments, a germanium channel quantum well semiconductordevice and its fabrication are described. In the following description,various embodiments will be described. However, one skilled in therelevant art will recognize that the various embodiments may bepracticed without one or more of the specific details, or with otherreplacement and/or additional methods, materials, or components. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of variousembodiments of the invention. Similarly, for purposes of explanation,specific numbers, materials, and configurations are set forth in orderto provide a thorough understanding of the invention. Nevertheless, theinvention may be practiced without specific details. Furthermore, it isunderstood that the various embodiments shown in the figures areillustrative representations and are not necessarily drawn to scale.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, material, orcharacteristic described in connection with the embodiment is includedin at least one embodiment that falls within the scope of the invention,but do not denote that they are necessarily present in every embodiment.Thus, the appearances of the phrases “in one embodiment” or “in anembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments. Various additional layers and/or structures may be includedand/or described features may be omitted in other embodiments.

Various operations will be described as multiple discrete operations inturn, in a manner that is most helpful in understanding the invention.However, the order of description should not be construed as to implythat these operations are necessarily order dependent. In particular,these operations need not be performed in the order of presentation.Operations described may be performed in a different order, in series orin parallel, than the described embodiment. Various additionaloperations may be performed and/or described operations may be omittedin additional embodiments.

FIG. 1 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 100, according to one embodimentof the present invention. In the illustrated embodiment, the device 100includes a substrate 102, which may be any material or materials onwhich the device 100 may be made. In some embodiments the substrate 102may be a substantially single-crystal silicon material, a substantiallysingle-crystal silicon material that is doped, a multi-crystal ormulti-layer substrate 102, or a semiconductor-on-insulator substrate102. The substrate 102 may not comprise silicon in some embodiments, butmay instead comprise a different substrate material, such as Ge, GaAs orInP. The substrate 102 may include one or more material(s), device(s),or layer(s), or may be a single material without multiple layers.

The substrate 102 surface on which the device 100 is to be formed mayhave a resistance between about 1 ohm and about 50,000 ohms percentimeter. The high resistivity may be achieved by a low dopantconcentration, lower than about 10¹⁶ carriers/cm³. In other embodiments,the substrate 102 may have different resistivities or the resistance maybe achieved by other methods.

There is a buffer region 104 on the substrate 102 in the illustratedembodiment. The buffer region 104 may function to accommodate for alattice mismatch between the substrate 102 and regions above the bufferregion 104 and to confine lattice dislocations and defects.

There is a lower barrier region 106 on the buffer region 104, a channelregion 108 on the lower barrier region 106, a first spacer region 110 onthe channel region 108, a doped region 112 on the spacer region 110, andan upper barrier region 114 on the doped region 112 in the illustratedembodiment.

The lower barrier region 106 may comprise a material with a higher bandgap than the material of which the channel region 108 is comprised. Thelower barrier region 106 comprises silicon and germanium in theillustrated embodiment, although in other embodiments it may compriseother materials such as InAlAs, InGaAs, GaAs, AlGaAs, InAlSb or InP(note that various materials similar to this may be used in otherregions of the device 100 such as the spacer regions and barrierregions). In embodiments where the lower barrier region 106 comprisesSiGe, various ratios of Si to Ge may be used. In an embodiment, thebottom barrier region comprises SiGe, so the ratio is 50% Si and 50% Ge.In other embodiments, the ratio may be chosen to impart a strain on thechannel region 108 or for other reasons. For example, in an embodimentthe bottom barrier region 106 comprises 70% SiGe and 30% Si. In anotherembodiment, the bottom barrier region 106 comprises between 60% and 80%SiGe and between 40% and 20% Si. In other embodiments, different ratiosmay be used.

The lower barrier region 106 may be doped or undoped. Any suitablemethod may be used to form the lower barrier region 106. In someembodiments, the lower barrier region 106 may have a thickness betweenabout one micron and three microns or less. In an embodiment the lowerbarrier region 106 has a thickness less than about 500 nanometers, inanother embodiment the lower barrier region 106 has a thickness of about100 nanometers, and in other embodiments it may have yet otherthicknesses.

The channel region 108 may be a quantum well channel region 108. Thequantum well channel region 108 comprises Ge. In an embodiment, thechannel region consists substantially of all Ge. In other embodiments,there may be other materials, such as Si, as part of the composition ofthe channel region 108. As mentioned above, the quantum well channelregion 108 may be strained by lattice size mismatch with the layers(such as the bottom barrier region 106) adjacent to the channel region108. In some embodiments, the channel region 106 is not strained. Anysuitable method may be used to form the quantum well channel region 108.In some embodiments, the quantum well channel region 108 may have athickness between about 3 nanometers and twenty nanometers, although itmay be less or more than that in other embodiments. In one embodiment,it has a thickness of about 10 nanometers.

The first spacer region 110 may comprise the same or a similar materialas the lower barrier region 106 in one embodiment, may compriseintrinsic undoped SiGe in another embodiment, and in other embodimentsit may comprise a different material. The first spacer region 110 may beformed by any suitable method. In one embodiment, the first spacerregion 110 has a thickness less than about 5 nanometers. In anotherembodiment, the first spacer region 110 has a thickness of about 2nanometers. In other embodiments, the first spacer 110 may have otherthicknesses.

The doped region 112 is on the spacer region 110 and is doped accordingto the design of the device 100 and the targeted threshold voltage ofthe device 100 in an embodiment. Note that the term “doped region 112”as used herein may be a modulation doped region, a delta-doped region oranother type of doped region in various embodiments.

The doped region 112 may comprise substantially the same material as thespacer region 110, with the addition of a dopant or dopants. Forexample, in an embodiment where the spacer region 110 comprises SiGe,the doped region 112 also comprises SiGe with the addition of a dopant.The dopant used in the doped region 116 may be boron or another p-typedopant. There may be a dopant density in the doped region 112 of betweenabout 1×10¹¹/cm² to about 8×10¹²/cm² in some embodiments, between aboutzero (undoped) and 5×10¹³/cm² in other embodiments, although differentdopant densities may be used in yet other embodiments. The density ofdopants may be chosen based by the device 100 design and targetedthreshold voltage of the device. In another embodiment, the doped region112 may comprise a different material than SiGe that is doped. In someembodiments, the doped region 112 may have a thickness of less thanabout 50 angstroms. In another embodiment, the doped region 112 has athickness of about 20 angstroms or less. In other embodiments, the dopedregion 112 may have other thicknesses.

There is an upper barrier region 114 on the doped region 112 in thedevice 100 illustrated in FIG. 1. The upper barrier region 114 maycomprise intrinsic undoped SiGe in an embodiment, silicon and germaniumin varying ratios in other embodiments, Si in another embodiment, othermaterials in other embodiments, and may consist of substantially thesame material as the lower barrier region 106 and/or the spacer region110 in yet other embodiments. The upper barrier region 114 may comprisea material with a larger band gap than the channel region 108 in anembodiment. The upper barrier region 114 may be formed by any suitablemethod. In one embodiment, the upper barrier region 114 has a thicknessless than about 5 nanometers. In another embodiment, the upper barrierregion 114 has a thickness of about 2 nanometers. In other embodiments,the upper barrier region 114 may have other thicknesses. This thicknessmay be chosen based on the targeted threshold voltage for the device100.

In an embodiment, the spacer region 110, doped region 112, and the upperbarrier region 114 may be formed with a continuous growth process. Forexample, the spacer region 110 can comprise SiGe and be formed in achamber. To form the doped region 112 the precursor flows are changed toinclude the dopant. To form the upper barrier region 114, the precursorflows are changed again to those used to form the spacer region 110. Inother embodiments, different ways to form the regions may be used andthe regions 110, 112, 114 may not have so similar a composition.

There is a gate dielectric 116 on the upper barrier region 114 and agate 118 on the gate dielectric 116 in the illustrated embodiment. Thegate dielectric 116 may comprise a material with a high dielectricconstant (high-k dielectric). The gate dielectric 116 may comprise amaterial with a high dielectric constant (high-k dielectric) such asAl₂O₃, although other materials such as La₂O₃, HfO₂, ZrO₂, TaO₅, orternary complexes such as LaAl_(x)O_(y), Hf_(x)Zr_(y)O_(z) or othermaterials may be used in other embodiments. In embodiments where thegate dielectric 116 is Al₂O₃, the Al₂O₃ may be deposited usingtrimethylaluminum (TMA) and water precursors with and ALD process in oneembodiment, although other methods to form it may be used. In anembodiment, the gate dielectric 116 has a thickness between about 0.7nanometers and 5 nanometers, in another embodiment the gate dielectric116 has a thickness less than 5 nanometers, and in other embodiments thegate dielectric 116 may have different thicknesses.

The gate 118 may comprise a metal-containing material such as Pt/Au,Ti/Au, Ti/Pt/Au, or another material or materials. In variousembodiments, the material or materials of the gate 118 may be chosen toprovide a desired workfunction. Source 120 and drain 122 regions may beformed adjacent the gate dielectric 116 and/or gate 118. In oneembodiment, the source and drain regions may comprise NiGeAu. In anotherembodiment, the source and drain regions may comprise TiPtAu. In otherembodiments, the source and drain regions 120, 122 may comprise anothermaterial or materials.

FIG. 2 is a cross sectional side view that illustrates a buffer region104 that is formed on the substrate 102 in one embodiment. The bufferregion 104 may function to accommodate for a lattice mismatch betweenthe substrate 102 and regions above the buffer region 104 and to confinelattice dislocations and defects. In the illustrated embodiment, thebuffer region 104 has multiple regions: a first buffer region 130 and asecond buffer region 132, although in other embodiments the bufferregion 104 may have different numbers of regions or simply be a singleregion.

In an embodiment the substrate 102 comprises silicon, the bottom barrierregion 106 comprises SiGe, and the first and second buffer regions 130,132 comprise silicon and germanium in different amounts. For example, inan embodiment the first buffer region 130 comprises 30% SiGe withsubstantially all the rest being Si, and the second buffer region 132comprises 70% SiGe with substantially all the rest being Si. Such anarrangement is a stepped buffer region 104, with discrete regions havingincreasing amounts of Ge. More than two stepped buffer regions and/ordifferent changes in material amounts may be present in otherembodiments.

In another embodiment there is a single graded buffer region 104 ratherthan multiple stepped buffer regions. The graded buffer region 104comprises Si_(x)Ge_(1-x), with x ranging between 1 (or another selectedstarting amount) at the bottom of the buffer region 104 adjacent thesubstrate 102 to 0.5 (or another selected ending amount) at the top ofthe buffer region 104 adjacent the bottom barrier region 106.

In yet another embodiment there is a single non-graded buffer region 104with a substantially homogenous SiGe (intrinsic or in a selected ratiowith Si or another material) composition all the way from the bottom ofthe buffer region 104 adjacent the substrate 102 to the top of thebuffer region 104 adjacent the bottom barrier region 106.

In embodiments with different materials in the substrate 102 and/orbottom barrier region 106, the buffer region 104 may also comprise adifferent material selected to form a relaxed top portion with reduceddefects compared to the bottom of the buffer region 104. For example, inan embodiment where the bottom barrier region 106 comprises GaAs, thetop portion of the buffer region 104 may comprise Ge, which issubstantially lattice-matched to GaAs. The buffer region 104 may thus begraded so there is increasing Ge and decreasing Si further from thesubstrate 102, may be stepped to have more Ge and less Si further fromthe substrate 102, or may simply be a layer of Ge. In other materialschemes, different buffer regions 104 with different materials andbuffering may be used.

The buffer region 104 (and any sub-regions 130, 132) may have sufficientthickness that most defects present at its bottom surface are notpresent at its top surface. Any suitable method may be used to form thebuffer region 104.

In some embodiments, the buffer region 104 may also include a nucleationregion between the first buffer region 130 and the substrate 102. Forexample, an embodiment may have a substrate 102 with an offcut vicinalsurface and a III-V material, such as GaAs, buffer region 104. Thenucleation region comprises gallium arsenide in one embodiment, althoughother materials such as GaSb or AlSb may be used in other embodiments.(Note that as used herein, when materials designated by their elementswithout subscripts, these designations encompass any mix of percentagesof the elements. For example, “InGaAs” encompasses In_(x)Ga_(1-x)As,with x ranging between zero (GaAs) and one (InAs). Similarly, InAlAsencompasses In_(0.52)Al_(0.48)As.) It is formed by molecular beamepitaxy (MBE), migration enhanced epitaxy (MEE), metal-organic chemicalvapor deposition (MOCVD), atomic layer epitaxy (ALE), chemical beamepitaxy (CBE), or another suitable method. It may be annealed afterformation to reduce dislocations. It has a thickness of less than about1000 angstrom in some embodiments, a thickness of less than about 500angstroms in some other embodiments. In one embodiment, the nucleationregion has a thickness of about 300 angstroms. In embodiments where thesubstrate 102 is a vicinal silicon material, the nucleation region maybe made sufficiently thick to fill all the terraces of the siliconsubstrate 102. In an alternative embodiment, other suitable nucleationregion materials or thicknesses may be used, or the nucleation regionmay be omitted.

Such a device with a Ge-based quantum well channel region 106 mayprovide a p-type transistor that provides better performance than othertypes of transistors.

FIG. 3 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 100, according to one embodimentof the present invention. The device 100 of FIG. 3 is similar to thedevice of FIG. 1, but with the doped region 112 and spacer region 110below the channel region 108 rather than above the channel region 108.This allows the channel region 108 to be closer to the gate 118 and mayprovide increased performance.

FIG. 4 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 100, according to one embodimentof the present invention. The device 100 of FIG. 3 is similar to thedevice of FIG. 1, but lacks the doped region 112 and spacer region 110in the material stack from which the device 100 is formed. Instead,there is a doped region 111 elsewhere in the device 100, which may beformed after the formation of the gate dielectric 116 and gate 118. Inthe illustrated embodiment, there are spacers on either side of the gate118 and a doped region 111 between the spacer and the contact stack. Thedoped region 111 in such a device may have different sizes, shapes andplacements, and there may or may not be spacers in such an alternativedevice. Forming a device 100 with a doped region 111 that is not ablanket layer as part of the material stack may simplify creation of thedevice 100.

FIG. 5 is a cross sectional side view that shows a material stack thatmay be used to form another embodiment of a germanium quantum wellchannel transistor device. This material stack includes a substrate 202,buffer region 204, lower barrier region 206, channel region 208, firstspacer region 210, doped region 212 and upper barrier region 214 thatmay be similar to the substrate 102, buffer region 104, lower barrierregion 106, channel region 108, spacer region 110, doped region 112 andupper barrier region 114, respectively, that are described above. Theillustrated embodiment also includes an etch stop region 240 on thefirst spacer region 210 and an upper spacer region 242 on the etch stopregion 240 and beneath the doped region 212.

In an embodiment, the spacer region 210 comprises SiGe, the etch stopregion 240 comprises silicon and is substantially free from germanium,and the upper spacer region 242 comprises SiGe. In some embodiments, theupper spacer region 242 and lower spacer region 210 consist ofsubstantially the same material, while in other embodiments there may bedifferences in composition of the two regions 210, 242. In someembodiments, the materials of the spacer regions 210, 242 and the etchstop region 240 are chosen to provide etch selectivity with a chosenetchant or etchants between the first spacer region 210 and the etchstop region 240 and between the etch stop region 240 and the upperspacer region 242. In an embodiment where the upper spacer 242 comprisesSiGe and the etch stop region 240 comprises Ge, potassium hydroxide maybe selected as an etchant that will remove the upper spacer region 242and stop at the etch stop region 240. Other etchants and/or materialsmay be used in other embodiments. In an embodiment, both the etch stopregion 240 and the lower spacer region 210 are thin. For example, theetch stop region 240 is ten angstroms thick or less and the first spacerregion 210 is also ten angstroms thick or less. In other embodiments,either or both of the etch stop region 240 and first spacer region 210may have greater thicknesses.

In the embodiment illustrated in FIG. 5, there is also a contact region244 on the upper barrier region 214. This contact region 244 may be usedto form source and drain contacts. In an embodiment, the contact region244 comprises a p+ doped SiGe material, which may have boron as thedopant at a density of between about 1×10¹⁹/cm² to about 1×10²²/cm² in.Other dopants, other concentrations, and other materials besides SiGemay be used in other embodiments.

FIG. 6 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 200, according to one embodimentof the present invention. This device 200 may be formed from thematerial stack illustrated in FIG. 5 and may also have a gate dielectric216 and gate 218 similar to those described with respect to the device100 of FIG. 1. The device 200 has a recessed gate 218. As there is etchselectivity between the upper spacer region 242 and the etch stop region240, the upper spacer region 242 (and other material above) may beremoved leaving only a thin etch stop region 240 and first spacer region210 between the channel region 208 and the gate dielectric 216. Inembodiments such as device 100 that lack an etch stop region 240 it maybe more difficult to get the channel region 108 as close to the gate 118as the channel region 208 in FIG. 6 is, and thus it may be easier toachieve better performance with the device 200 of FIG. 6.

Additionally, it may be difficult to form a high-k gate dielectric 216on SiGe. The presence of an etch stop region 240 that comprises adifferent material such as silicon may allow formation of a high-k gatedielectric 216 much more easily than if the device 200 lacked the etchstop region 240.

Adjacent the gate dielectric 216 and gate 218 are additional regionsthat are not beneath the gate dielectric 216 and gate 218. In theillustrated embodiment, these additional regions include the remainingportions of the upper spacer region 242, a doped region 212 on the upperspacer region 242, remaining portions of the upper barrier region 214 onthe doped region 212, and remaining portions of the contact region 244on the upper barrier region 214. These additional regions may beconsidered a contact stack, with a contact stack being shown on eitherside of the gate dielectric 216 and gate 218 in FIG. 6.

FIG. 7 is a cross sectional side view that illustrates the etch stopregion 240 present in some embodiments. As seen in FIG. 7, the etch stopregion 240 may include multiple regions. In an embodiment, the etch stopregion 240 may include a first silicon region 246 and a second silicondioxide region 248. The second silicon dioxide region 248 may be formedby oxidizing silicon of the etch stop region 240 after removal of aportion of the upper spacer region 242 above the etch stop region 240 toexpose the portion of the etch stop region 240. As illustrated in FIG.7, only an upper portion of the etch stop region 240 is oxidized, but inother embodiments, the whole thickness of the etch stop region 240 maybe oxidized so that the etch stop region 240 under the gate 218 lacks asilicon region 246. Such a silicon dioxide region 248 may be formed if,for example, it would be easier to form the desired gate dielectric 216material on silicon dioxide than on silicon. In other embodiments,different alterations to some or all of the etch stop region 240 beneaththe gate 218 may be done by the addition, subtraction, or altering ofmaterial.

FIG. 8 is a cross sectional side view that illustrates what portion ofthe etch stop region 240 may be altered in an embodiment. Regions 240 amay remain silicon, while the region 240 b that will be under the gate218 is altered as described above. As described above, the entirethickness of region 240 b may be altered (for example converted fromsilicon to silicon dioxide) or only a portion of the thickness. In otherembodiments, the boundaries between the altered 240 b and unaltered 240a regions of the etch stop region 240 may be in different places, and insome embodiments, the etch stop region 240 may not have an alteredregion 240 b at all.

FIG. 9 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 200, according to anotherembodiment of the present invention. This device 200 is similar to thedevice 200 of FIG. 6, but has the spacer region 210 and doped region 212beneath the channel region 208 rather than above the channel region 208.In such an embodiment, the doped region 212 extends laterally so itexists below the gate 218 and does not end at the edge of the contactstacks.

In yet another embodiment (not pictured), the device is similar to thedevice 200 of FIG. 9, but lacks the doped region 212 and spacer region210. Instead, there is a doped region elsewhere in the device, similarto the doped region 111 in the device 100 pictured in FIG. 4.

FIG. 10 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 300, according to anotherembodiment of the present invention. In the device 300 of FIG. 10, oneor more of the buffer region 304, barrier regions 306, 314, spacerregion 310, or doped region 312 comprise a group III-V material whilethe channel region 308 comprises (or substantially consists of) Ge. Agroup III-V material includes a group III element and a group V element,such as gallium arsenide (GaAs), indium antimonide (InSb), indiumphosphide (InP), and indium gallium arsenide (InGaAs).

For example, in an embodiment, the buffer region 304 comprises a groupIII-V material, the bottom barrier region 306, spacer region 310, andupper barrier region 314 comprise undoped GaAs, the channel regioncomprises Ge, and the doped region 312 comprises GaAs doped with Be, C,or Si. Such a device may provide better band offsets than if materialsother than group III-V materials are used in those regions 304, 306,310, 312. AlAs, AlGaAs, or other group III-V materials may be used inplace of GaAs in some or all of the regions 304, 306, 310, 312.

In embodiments where the buffer region 304 comprises a group III-Vmaterial, the buffer region 304 may include multiple subregions 330,332, as seen in FIG. 11. The substrate 302 may comprise high-resistivityp-type or n-type vicinal silicon material having regular arrays ofdouble-stepped (100) terraces across the substrate surface in someembodiments. A vicinal surface may be prepared by offcutting thesubstrate 302 from an ingot. In some embodiments, the (100) substratesurface is offcut at an angle between 2 and 8 degrees towards the [110]direction. In a particular embodiment, the (100) substrate surface isoffcut at an angle of about 4 degrees towards the [110] direction. Avicinal surface is a higher order crystal plane of the silicon substrate302, such as, but not limited to the (211), (511), (013), (711) planes.In other embodiments, the substrate 302 may not have been offcut or havea vicinal surface.

The substrate 302 surface on which the device 300 is to be formed mayhave a resistance between about 1 ohm and about 50,000 ohms percentimeter. The high resistivity may be achieved by a low dopantconcentration, lower than about 10¹⁶ carriers/cm³. In other embodiments,the substrate 302 may have different resistivities or the resistance maybe achieved by other methods.

In some embodiments the substrate 302 may be a substantiallysingle-crystal silicon material, a substantially single-crystal siliconmaterial that is doped, a multi-crystal or multi-layer substrate 302. Invarious embodiments, the substrate 302 could comprise germanium,germanium on silicon, or could be a silicon-on-insulator substrate 302.The substrate 302 may not comprise silicon in some embodiments, but mayinstead comprise a different material, such as a different semiconductoror Ge or a group III-V material such as GaAs or InP. The substrate 302may include one or more material(s), device(s), or layer(s), or may be asingle material without multiple layers.

The buffer region 304 on the substrate 302 may function to accommodatefor a lattice mismatch between the substrate 302 and regions above thebuffer region 304 and to confine lattice dislocations and defects. Inthe illustrated embodiment, the buffer region 304 has multiple regions:a nucleation region 330 and a first buffer region 332, although in otherembodiments the buffer region 304 may have different numbers of regionsor simply be a single region.

The nucleation region 330 comprises gallium arsenide in one embodiment,although other materials such as GaSb or AlSb may be used in otherembodiments. (Note that as used herein, when materials designated bytheir elements without subscripts, these designations encompass any mixof percentages of the elements. For example, “InGaAs” encompassesIn_(x)Ga_(1-x)As, with x ranging between zero (GaAs) and one (InAs).Similarly, InAlAs encompasses In_(0.52)Al_(0.48)As.) It is formed bymolecular beam epitaxy (MBE), migration enhanced epitaxy (MEE),metal-organic chemical vapor deposition (MOCVD), atomic layer epitaxy(ALE), chemical beam epitaxy (CBE), or another suitable method. It maybe annealed after formation to reduce dislocations. It has a thicknessof less than about 1000 angstrom in some embodiments, a thickness ofless than about 500 angstroms in some other embodiments. In oneembodiment, the nucleation region 330 has a thickness of about 300angstroms. In embodiments where the substrate 302 is a vicinal siliconmaterial, the nucleation region 330 may be made sufficiently thick tofill all the terraces of the silicon substrate 302. In an alternativeembodiment, other suitable nucleation region 330 materials orthicknesses may be used, or the nucleation region 330 may be omitted.

On the nucleation region 330 is a first buffer region 332 in theillustrated embodiment. In an embodiment, the first buffer region 332comprises a GaAs material, although other materials, such as InAlAs,AlSb, or other materials may be used. In an embodiment, the first bufferregion 332 consists substantially the same material as the nucleationregion 330. The buffer region 332 may also be formed by molecular beamepitaxy (MBE), migration enhanced epitaxy (MEE), metal-organic chemicalvapor deposition (MOCVD), atomic layer epitaxy (ALE), chemical beamepitaxy (CBE), or another suitable method. The first buffer region 332may have a thickness of less than one micron, between 0.3 microns andone micron, about 0.3 micron, or another thickness in variousembodiments.

The first buffer region 332 may be formed by the same process used toform the nucleation region 330 in some embodiments. In such anembodiment, the growth of the first buffer layer 332 may be performed ata higher temperature than that used for the nucleation layer 304. Whilefirst buffer region 332 may be considered and is shown as a separateregion than nucleation region 330, both regions 330, 332 may beconsidered buffers, with region 332 thickening the III-V buffer regionstarted by nucleation region 330. The film quality of region 332 may besuperior to that of the nucleation region 332 because it may be formedat a higher growth temperature. Also, during the formation of region332, the flux rate can be relatively high because the polar nucleationregion 330 may eliminate danger of anti-phase domains (APD) formation.

The buffer region 332 may be a single thick non-graded buffer region332, a multi-stepped buffer region 332, a graded buffer region 332, oranother form of buffer, similar to the buffer region 104 described withrespect to FIG. 2.

Note that some embodiments may lack a nucleation region 330 and/or abuffer region 332. For example, in embodiments where the substrate 302comprises a group III-V material, the device 300 may lack nucleationregion 330 and/or buffer region 332. In an embodiment where the bufferregion 304 comprises SiGe, the buffer region 304 may lack a nucleationregion 330 and be similar to the buffer region 104 described withrespect to FIG. 2.

Returning to FIG. 10, in another embodiment the device 300 has a bufferregion 304 similar to buffer region 104 and comprising SiGe and a lowerbarrier region 306 comprising SiGe with a channel region 308 thatcomprises Ge and an upper barrier region 314 that comprises GaAs oranother group III-V material. The SiGe of the lower barrier region 306(which may be in direct contact with the channel region 308) can strainthe channel region 308 while the group III-V material of the upperbarrier region 314 provides improved band offset.

In another embodiment, the bottom barrier region 306 comprises a groupIII-V material, the buffer region 304 comprises SiGe or a group III-Vmaterial, the channel region 308 comprises Ge, and the upper barrierregion 314 comprises SiGe. Various other combinations of SiGe and groupIII-V materials in the barrier regions 306, 314, spacer region 310, andbuffer region 304 may also be used. Also, embodiments with the dopedregion 312 and spacer region 310 being under the channel region 308 (asseen in FIG. 3) or that lack a blanket doped region 312 and instead havealternative doped regions (as doped regions 111 seen in FIG. 4) are alsopossible. In addition, the etch stop region 240 and additional spacerregion 242 of FIGS. 5-9 may also be used in embodiments of a device 300that has group III-V material as one or more of the buffer region 304,barrier regions 306, 314, spacer region 310, or doped region 312.

FIG. 12 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 400 that has one or morenon-homogenous barrier 406, 414 and/or spacer 410 regions, according toanother embodiment of the present invention. Rather than have ahomogenous material composition through the thickness of the region(s)406, 414, and/or 410, the material is non-homogenous to be transitionedto be more similar to the channel region 408 closer to the channelregion 408 and less similar to the channel region 408 further from thechannel region 408.

FIG. 12 a is a cross sectional side view similar to FIG. 12, butincludes a non-graded spacer region 411 between the graded spacer region410 and the doped region 412. In some embodiments, there may be such anon-graded, relatively homogenous spacer region 411 between the gradedspacer region 410 and the doped region 412. The spacer region 411 mayconsist substantially of the same material as the top portion of thegraded spacer region 410 in some embodiments, although in otherembodiments, it may have a different composition.

For example, in an embodiment, the bottom 456 of the lower barrierregion 406 comprises SiGe. The channel region 408 consists substantiallyof germanium. The lower barrier region 406 is graded so that there ismore and more germanium until at the top 458 of the lower barrier region406 it consists substantially of germanium. Similarly, the spacer region410 is substantially germanium at the bottom 460. The spacer region 410is graded so that there is less germanium and more silicon towards thetop 462 of the spacer region 410.

FIGS. 13 and 14 are graphs that illustrate a graded bottom barrierregion 406 and spacer region 410 according to one embodiment of thepresent invention. FIG. 13 is a graph that illustrates the compositionof the bottom barrier region 406 between its bottom 456 and top 458, andFIG. 14 is a graph that illustrates the composition of the spacer region410 between its bottom 460 and top 462. As seen in FIG. 13, the bottombarrier 406 comprises SiGe at its bottom 456, with more Ge and less Sifurther towards the top 458, until at the top 458 the bottom barrier 406is substantially all Ge (in this embodiment, the channel region 408 issubstantially all Ge—in other embodiments, the top 458 of the barrierregion 406 reaches a composition substantially the same as thecomposition of the channel region 408). FIG. 14 shows the reverse ofthis with respect to the spacer region 410: the spacer region 410 issubstantially all Ge at the bottom 460 with more Si and less Ge furthertowards the top 462 until at the top 462 the spacer region 410 is SiGe.

FIGS. 15 and 16 are graphs that illustrate a graded bottom barrierregion 406 and spacer region 410 according to one embodiment of thepresent invention. FIG. 15 is a graph that illustrates the compositionof the bottom barrier region 406 between its bottom 456 and top 458, andFIG. 16 is a graph that illustrates the composition of the spacer region410 between its bottom 460 and top 462. As seen in FIG. 15, the bottombarrier 406 comprises SiGe at its bottom 456 and remains at the samecomposition for a portion of its thickness. Then, partway through thethickness of the bottom barrier 406, the Si starts to decrease and Geincrease, until at the top 458 the bottom barrier 406 is substantiallyall Ge. FIG. 16 shows the reverse of this with respect to the spacerregion 410: the spacer region 410 is substantially all Ge at the bottom460 with more Si and less Ge further towards the top 462 until it hits aselected ratio of Si to Ge. It continues at this selected compositionuntil the top 462. (Note that the embodiment illustrated in FIG. 12 acorresponds to the graph of FIG. 16—the portion that has a relativelyhomogenous selected ratio of Si to Ge may be the spacer region 411rather than a portion of the graded spacer region 410. Similarly, thebottom portion of bottom barrier 406 may be considered simply a portionof the bottom barrier 406 or may be considered a different relativelyhomogenous region.)

FIGS. 17 and 18 are graphs that illustrate a graded bottom barrierregion 406 and spacer region 410 according to another embodiment of thepresent invention. FIG. 17 is a graph that illustrates the compositionof the bottom barrier region 406 between its bottom 456 and top 458, andFIG. 16 is a graph that illustrates the composition of the pacer region410 between its bottom 460 and top 462. As seen in FIG. 17, the bottombarrier 406 comprises SiGe at its bottom 456 and remains at the samecomposition for a portion of its thickness. Then, partway through thethickness of the bottom barrier 406, the Si starts to decrease and Geincrease, until at the top 458 the bottom barrier 406 has a selectedcomposition. In this embodiment the selected composition at the top 458is not the same as the composition of the channel region 408. FIG. 18shows the reverse of this with respect to the spacer region 410: thespacer region 410 has a first composition somewhat different from thecomposition of the channel region 408 at the bottom 460, and thecomposition of the spacer region 410 becomes less similar to thecomposition of the channel region 408 further towards the top 462 untilit hits a selected composition and continues at this selectedcomposition until the top 462.

FIGS. 19 and 20 are graphs that illustrate a stepped bottom barrierregion 406 and spacer region 410 according to another embodiment of thepresent invention. In contrast to the embodiments shown in FIGS. 13-18,the bottom barrier region 406 and spacer region 410 have discrete stepsof changed composition, rather than a relatively smooth gradation. FIG.19 is a graph that illustrates the composition of the bottom barrierregion 406 between its bottom 456 and top 458, and FIG. 20 is a graphthat illustrates the composition of the pacer region 410 between itsbottom 460 and top 462. As seen in FIG. 19, the bottom barrier 406comprises SiGe at its bottom 456 and remains at the same composition fora portion of its thickness. The amount of Si increases and Ge decreasesin steps rather than smoothly, until at the top 458 the bottom barrier406 has a selected composition. FIG. 20 shows the reverse of this withrespect to the spacer region 410: the spacer region 410 has decreasingGe and increasing Si in stepped increments until it hits a selectedcomposition and continues at this selected composition until the top462.

While the examples in FIGS. 13 through 20 show SiGe and Ge, these sametwo compositions were consistently used in each Figure only for clarity.This does not indicate that they are the materials used in all possibleembodiments. Different materials in addition to or in place of Si and Gemay be used under the same non-homogenous concept. For example, thebottom barrier region 406 may comprises 60% SiGe and 40% Si at thebottom 456 and include more and more Ge as one moves towards the top458, which may be substantially all Ge or still include some Si. Also,materials other than Si and/or Ge may be used.

Note that while FIGS. 15 and 16 appear to show only a small portion ofthe thickness of the bottom barrier 406 and spacer 410 having the SiGecomposition, the graphs are not to scale, and it may be that themajority of the thicknesses of the bottom barrier 406 and spacer 410having a homogenous SiGe composition, with a small portion of theirthicknesses including a graded composition change. For example, 80%, 90%or more of the thicknesses of the bottom barrier 406 and spacer 410 maycomprise a homogenous material (in the illustrated case, SiGe), althoughin other embodiments different percentages of the bottom barrier 406 andspacer 410 may be homogenous. This is true for the embodimentsillustrated in the other graphs of FIGS. 13-20 as well—the graphs arenot to scale but merely illustrate that a portion of the thicknesses ofthe bottom barrier 406 and spacer 410 may form a non-homogenoustransition between material composition of the portions of the bottombarrier 406 and spacer 410 further from the channel region 408 to have acomposition more similar to the channel region 408 at locations of thebottom barrier 406 and spacer 410 closer to the channel region 408, sothat the difference between the composition of the channel region 408and regions immediately adjacent the channel region 408 are not sogreat.

By having more gradual changes in composition at the boundaries betweenthe bottom barrier region 406 and channel region 408 and between thechannel region 408 and the spacer region 410, the carriers of the device400 may be better located in the middle of the channel region 408 ratherthan at an abrupt interface between dissimilar materials of the channelregion 408 and a spacer 410 or barrier 406 adjacent the channel region408, which may improve device performance.

While FIG. 12 illustrated a device 400 with a doped region 412 above thechannel region 408, the device 400 may have other arrangements whileretaining the use of a graded or stepped transition from a firstselected composition of the barrier region 406 or spacer region 410 to acomposition more similar to the channel region 408 at locations closerto the channel region 408. Embodiments with the doped region 412 andspacer region 410 being under the channel region 408 (similar to thedevice seen in FIG. 3, in which case the upper barrier 414 would have anon-homogenous transition portion rather than the lower barrier 406) orthat lack a blanket doped region 412 and instead have alternative dopedregions (similar to doped regions 111 seen in FIG. 4, in which case theupper barrier 414 would have a transitional portion instead of thespacer 410) are also possible. In addition, the etch stop region 240 andadditional spacer region 242 of FIGS. 5-9 may also be used inembodiments of a device 400 that has less abrupt material transitionsbetween the channel region 408 and immediately adjacent regions. Also,one or more of the spacer 410, bottom barrier 406, or top barrier 414regions may comprise a group III-V material as is described with respectto FIGS. 10 and 11.

FIG. 21 is a cross sectional side view that illustrates a germaniumquantum well channel transistor device 500 that has multiple gatedielectric regions 570, 572, according to another embodiment of thepresent invention. In some cases a desired gate dielectric material mayhave compatibility issues with the material on which the gate dielectricis to be formed or other problems. Rather than avoiding that desiredgate dielectric material entirely, the gate dielectric may comprisemultiple gate dielectric regions 570, 572. While FIG. 21 shows two gatedielectric regions 570, 572, more than two may be used in otherembodiments.

For example, it may be advantageous to use HfO₂ as a gate dielectricmaterial. However, having HfO₂ in contact with a Ge or SiGe-containingupper barrier region 514 or other region immediately below the gatedielectric may result in various problems that negatively affect thedevice 500. To avoid these problems, a first gate dielectric 570 ofhafnium silicate may be formed, followed by a second gate dielectric 572of HfO₂. Other materials such as aluminum oxide, TaSiO, TaSiON, La₂O₃,or other materials can be used for the first gate dielectric region 570.Other materials such as ZrO₂, Ti₂O₅, Ta₂O₅, HfSiON, HfSiO, or othermaterials can be used for the second gate dielectric region 572. Thesecond gate dielectric 572 may have a higher dielectric constant thanthe first dielectric region 570 in some embodiments, although this isnot necessary.

The multiple dielectric regions 570, 572 of FIG. 21 may be applicable toany of the other embodiments of devices described herein. The dopedregion 512 may be above the channel region 508, below the channel region508, or elsewhere. The regions immediately adjacent the channel region508 may have varying composition to avoid an abrupt transition to thematerial composition of the channel region 508. The etch stop region 240and additional spacer region 242 of FIGS. 5-9 may also be used withmultiple gate dielectric regions 570, 572. Also, one or more of thespacer 510, bottom barrier 506, or top barrier 514 regions may comprisea group III-V material as is described with respect to FIGS. 10 and 11.

FIG. 22 is a cross sectional side view that illustrates a device 600with a germanium quantum well channel region 608 transistor (on the leftside of FIG. 22) and a group III-V material quantum well channel region708 transistor (on the right side of FIG. 22) on the same substrate 602,according to one embodiment of the present invention. In someembodiments, germanium quantum well channel transistors, such as thoseshown and described in FIGS. 1-21, may be used as PMOS transistors whileother types of transistors, such as the group III-V material quantumwell channel region 708 transistor, may be used as NMOS transistors.These combinations of different types of transistors may make up CMOScircuitry in various different systems, such as a computer centralprocessing unit.

In the embodiment shown in FIG. 22, the germanium quantum well channelregion 608 transistor may have any structure as described herein. Onesuch structure is a substrate 602, buffer region 604, bottom barrierregion 606, channel region 608 consisting substantially of germanium,spacer region 610, doped region 612, upper barrier region 614, gatedielectric 616, gate electrode 618, source region 620 and drain region622. There may be an isolation region 680 between the germanium quantumwell channel region 608 transistor and the group III-V material quantumwell channel region 708 transistor.

The group III-V material quantum well channel region 708 transistor maycomprise many different types of group III-V material quantum wellchannel region 708 transistors. In one embodiment, it includes a bufferregion 704 (which may include a nucleation region) comprising GaAs, alower barrier region 706 comprising InAlAs, a channel region 708comprising InGaAs, a spacer region 710 comprising InAlAs, a doped region712 comprising doped InAlAs, an upper barrier region 714 comprisingInAlAs, a gate dielectric 716, a gate 718, a source region 720, and adrain region 722. In other embodiments, different materials and/orstructures may be used with the group III-V material quantum wellchannel region 708 transistor.

As, in some embodiments, a germanium quantum well channel transistor mayprovide better PMOS performance, and a group III-V material quantum wellchannel transistor may provide better NMOS performance, the use of bothtypes of transistors on a single substrate 602 may provide betteroverall device 600 performance than if just one type of transistor wereused for both NMOS and PMOS transistors.

FIG. 23 is a cross sectional side view that illustrates a device 800with a germanium quantum well channel region 608 transistor (on the leftside of FIG. 23) and a non-quantum well channel transistor (on the rightside of FIG. 23, also referred to as a “standard” transistor) on thesame substrate 602, according to one embodiment of the presentinvention. In some embodiments, germanium quantum well channeltransistors, such as those shown and described in FIGS. 1-21, may beused as PMOS transistors while other types of transistors, such wellknown silicon transistors that have been widely used for decades, may beused as NMOS transistors. These combinations of different types oftransistors may make up CMOS circuitry in various different systems,such as a computer central processing unit.

In the embodiment shown in FIG. 23, the germanium quantum well channelregion 608 transistor may have any structure as described herein. Onesuch structure is a substrate 602, buffer region 604, bottom barrierregion 606, channel region 608 consisting substantially of germanium,spacer region 610, doped region 612, upper barrier region 614, gatedielectric 616, gate electrode 618, source region 620 and drain region622. There may be an isolation region 780 between the germanium quantumwell channel region 608 transistor and the group III-V material quantumwell channel region 708 transistor. Trenches may have been formed in thesubstrate 602 and then filled in by the regions of the germanium quantumwell channel region 608 transistor to result in the device 800 shown inFIG. 23. In other embodiments, such trenches may be avoided, in whichcase the germanium quantum well channel region 608 transistor may extendabove the non-quantum well channel transistor.

The standard transistor may take one of many different structures anduse many different materials, as is known in the art. In one embodiment,it includes a gate dielectric 804 and a gate electrode 806 on the gatedielectric 804. Adjacent sidewalls 808 of the gate dielectric 804 andgate electrode 806 are spacers 810. In the substrate 602 on either sideof the gate dielectric 804 and gate electrode 806 are a source region812 and a drain region 814. Between the source region 812 and drainregion 814 in the substrate 602 is a channel region 816. The standardtransistor may have a silicon dioxide gate dielectric 804 andpolysilicon gate 806, a high-k gate dielectric 804 and a metal gate 806,may be a planar or multigate transistor, and take various other formsand use various materials.

As, in some embodiments, a germanium quantum well channel transistor mayprovide better PMOS performance, and a standard transistor may providebetter NMOS performance, the use of both types of transistors on asingle substrate 602 may provide better overall device 800 performancethan if just one type of transistor were used for both NMOS and PMOStransistors.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. This description and the claims following include terms, suchas left, right, top, bottom, over, under, upper, lower, first, second,etc. that are used for descriptive purposes only and are not to beconstrued as limiting. For example, terms designating relative verticalposition refer to a situation where a device side (or active surface) ofa substrate or integrated circuit is the “top” surface of thatsubstrate; the substrate may actually be in any orientation so that a“top” side of a substrate may be lower than the “bottom” side in astandard terrestrial frame of reference and still fall within themeaning of the term “top.” The term “on” as used herein (including inthe claims) does not necessarily indicate that a first layer “on” asecond layer is directly on and in immediate contact with the secondlayer unless such is specifically stated; there may be a third layer orother structure between the first layer and the second layer on thefirst layer. The embodiments of a device or article described herein canbe manufactured, used, or shipped in a number of positions andorientations. Persons skilled in the relevant art can appreciate thatmany modifications and variations are possible in light of the aboveteaching. Persons skilled in the art will recognize various equivalentcombinations and substitutions for various components shown in theFigures. It is therefore intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

We claim:
 1. A device, comprising: a lower barrier region comprising alarge band gap material; a quantum well channel region comprisinggermanium on the lower barrier region; an upper barrier regioncomprising a large band gap material on the quantum well region; a gatedielectric on the quantum well channel region and not in contact withthe quantum well channel region; a gate electrode on the gatedielectric; and wherein at least one of the lower barrier region or theupper barrier region comprises a group III-V material.
 2. The device ofclaim 1, wherein the lower barrier region comprises a group III-Vmaterial.
 3. The device of claim 2, wherein the lower barrier regioncomprises GaAs.
 4. The device of claim 1, wherein one of the lowerbarrier region or the upper barrier region comprises silicon germaniumand does not comprise a group III-V material.
 5. The device of claim 1,wherein the lower barrier region comprises silicon germanium and theupper barrier region comprises a group III-V material.
 6. The device ofclaim 1, wherein both the lower barrier region and the upper barrierregion comprise a group III-V material.
 7. The device of claim 1,further comprising: a spacer region on the quantum well channel regionand below the upper barrier region; and a doped region on the spacerregion and below the upper barrier region.
 8. The device of claim 7,wherein the spacer region, the lower barrier region, and the upperbarrier region all comprise III-V material.
 9. The device of claim 8,wherein the doped region comprises a doped III-V material.
 10. Thedevice of claim 1, further comprising: a spacer region under the quantumwell channel region and on the lower barrier region; and a doped regionunder the spacer region and on the upper barrier region.
 11. A device,comprising: a lower barrier region comprising a large band gap material;a quantum well channel region comprising germanium on the lower barrierregion; an upper barrier region comprising a large band gap material onthe quantum well region; a gate dielectric on the quantum well channelregion and not in contact with the quantum well channel region; a gateelectrode on the gate dielectric; and wherein the lower barrier regioncomprises silicon germanium, with there being a higher percentage ofsilicon in a portion of the lower barrier region further from thequantum well channel region and a lower percentage of silicon in aportion of the lower barrier region closer to the quantum well channelregion.
 12. The device of claim 11, wherein the decrease in siliconpercentage in the lower barrier region is substantially smooth.
 13. Thedevice of claim 11, wherein the decrease in silicon percentage in thelower barrier region is stepped.
 14. The device of claim 11, furthercomprising: a spacer region on the quantum well channel region and belowthe upper barrier region; and a doped region on the spacer region andbelow the upper barrier region; and wherein the spacer region comprisessilicon germanium, with there being a higher percentage of silicon in aportion of the spacer region further from the quantum well channelregion and a lower percentage of silicon in a portion of the spacerregion closer to the quantum well channel region.
 15. A device,comprising: a lower barrier region comprising a large band gap material;a quantum well channel region comprising germanium on the lower barrierregion; an upper barrier region comprising a large band gap material onthe quantum well region; a first dielectric material having a firstdielectric constant on the quantum well channel region; a seconddielectric material having a second dielectric constant on the firstdielectric material, the second dielectric constant being higher thanthe first dielectric constant; and a gate electrode on the seconddielectric material.
 16. The device of claim 1, wherein the firstdielectric material comprises a material selected from the groupconsisting of HfSiO, Al₂O₃, TaSiO, TaSiON, and La₂O₃.
 17. The device ofclaim 1, wherein the second dielectric material comprises a materialselected from the group consisting of HfO₂, ZrO₂, Ti₂O₅, Ta₂O₅, HfSiON,and HfSiO.